Lemma 2.5 (Postprocessing of zCDP [1]}) 

Let \(\mathcal {M}: \mathcal {X}^n \rightarrow \mathcal {Y}\)  and
\(f: \mathcal {Y} \rightarrow \mathcal {Z}\)  such that \(\mathcal {M}\) 
satisfies \(\rho \) -zCDP. Define \(\mathcal {M}^{\prime }: \mathcal {X}^n \rightarrow \mathcal {Z}\) 
such that \(\mathcal {M}^{\prime }(x) = f \left( \mathcal {M}(x) \right)\) .
Then \(\mathcal {M}^{\prime }\)  satisfies \(\rho \) -zCDP.

